Removing unwanted dc overshoot from PS

Discussion in 'The Projects Forum' started by EEDude, Oct 9, 2009.

  1. EEDude

    Thread Starter Active Member

    Nov 18, 2008
    I am using a Keithly power supply sourcing DC current. The devices I am trying to measure are becoming damaged, I believe due to some overshoot when the meter transitions from low to high current. What kind of circuit can I put in order to smooth out the transition from low to high?? High pass filter?? Typically I am trying to measure a device around 120, 125 ohms at 125mV. So typically I am sourcing around 1mA of current. Thanks for your help!!
  2. steveb

    Senior Member

    Jul 3, 2008
    Please explain what you are doing in more detail with a schematic. Is the Keithly power supply a voltage source or a current source? What is the device you are testing? Is it a sensitive device that is easily damaged?

    Without details, it sounds like you are switching a multimeter from low to high current scale. This may be breaking and then making the circuit which is not a good thing to do and will result in transients as you described. A simple solution is to not use a current meter, but instead insert a current sensing resistor in series and measure the voltage drop across it. Bring the voltage or current source up to full power slowly to prevent (or at least minimize) transients.
  3. EEDude

    Thread Starter Active Member

    Nov 18, 2008
    Yes sourcing current. The devices are sensitive, they are tiny resistors printed onto a silicon wafer. So basically the circuit is just a current source in series with a resistor. Software and electromechanical equipment automates the process of sourcing, contacting the test leads, and recording the measurements at different magnetic field intervals. Need to source current because it saves time for this application since it creates less measuremens for the meters to have to take per device. When the current supply is triggered I believe it is damaging some devices, so I am wondering if it is possible to ramp up to the current level with some sort of filter or diode circuit? Thanks
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Is the supply left on while you are moving the probes to a new test site? That would be bad, because it goes to full output voltage into an open circuit. Shutting off the current source, and then enabling it after the probes have moved to a new test site has the opposite problem, when it overshoots and rings while settling down.

    I would leave the current source on always, and use a relay to "load" the current source with a resistor close to the nominal value of what you will be measuring. Connect the probes to the new test site, and then open the relay. That way, the load change on the current source is constrained to be no more than a factor of two, so it will minimize the gyrations it goes through in settling to its new voltage.

    Of course, it is also possible to fix the current source. It is, after all, a feedback system. By using a PID type of feedback controller, it should be possible to control the system damping so it is "overdamped", and thereby approaches the set point from one side without overshooting.